The present invention relates to a positive column type plasma treatment apparatus having a novel container or substrate support (herein after referred to as "susceptor") structure.
A plasma treatment is a method for creating plasma and very active radicals by applying energy such as electromagnetic wave to a certain substance and for forming a film on a substrate by contacting the radicals to the substrate and for implementing plasma treatments such as etching and ashing of the material on the surface of the substrate. A plasma treatment apparatus refers to equipments in general for implementing those treatments.
Such a plasma treatment apparatus comprises a plasma reaction chamber which is a vacuum chamber provided with means for introducing and discharging reaction gas for producing plasma, a means for supplying energy such as electromagnetic wave for turning the original gas introduced in the plasma treating chamber into plasma state, a substrate to be treated by the plasma and its supporting means, and a heating means for warming up the substrate as necessary.
By the way, such a plasma treatment largely depends on activity of radicals and a degree of plasma treatment on the substrate to be treated is largely changed depending on a condition of plasma discharge for generating the radicals and on a distribution of density of the radicals. What is necessary in treating plasma uniformly on a large area is to select such a plasma discharge condition that generates a large amount of radicals uniformly across the large area.
As electromagnetic wave energy for turning such an reaction gas into plasma, 13.56 MHz high frequency wave, micro wave, D.C. or low frequency electromagnetic wave energy is used. Normally plasma discharge by high frequency wave is often used.
There are two types in such plasma discharge; an inductive coupling type and a capacitive coupling type. The inductive coupling type is so called a non-polar discharge. Among the capacitive coupling type plasma discharges, a plane parallel plate electrode type plasma discharge is well known. As an example of the plane parallel plate electrode type plasma treatment apparatus, a schematic drawing of a plasma CVD apparatus is shown in FIG. 1.
As seen in the figure, a pair of plane parallel plate electrodes 7 and 1 are disposed within a reaction chamber 6 and are connected to a high frequency power source 2, respectively. A substrate to be treated 3 is placed on one electrode and is heated by a heating means 4 such as a heater.
Reaction gas is introduced into the reaction chamber from a gas inlet 5, is decomposed and activated by high frequency power supplied to the electrodes a film is formed on the substrate 3. To implement plasma treatment (forming a film) uniformly, an uniform plasma discharge needs to be realized. Sometimes the electrode on which the substrate is placed is rotated to supplement the uniformity, but it has such disadvantages that it complicates the apparatus and increases the size thereof.
Moreover, the substrate is normally placed on the cathode or anode of the plane parallel plate electrode or in the cathode dark space or anode dark space which are created near them when plasma treatment is performed, so that a size of an area to be treated cannot be increased more than the size of the electrode.
Furthermore, sometimes such a phenomenon that reaction products adhere flakily on the inner wall of the reaction chamber other than the substrate occurs and the reaction products fall on the substrate when a film is formed, thereby causing a problem that pinholes are created on the thin film.
By the way, such technologies as plasma CVD and plasma etching are widely being put into practical use in manufacturing semiconductor devices and electronic parts and many equipments for mass-producing them are being proposed. Lately in particular, diameters of substrate wafers for semiconductor devices and sizes of substrates are being increased more and more. Especially an active matrix type liquid crystal electro-optic device in which this film transistors are formed as switching elements for the liquid crystal electro-optic device is remarkably increasing in its size and it has become necessary to form a semiconductor thin film for the thin film transistors and to perform etching on a substrate with a size of 15 to 20 inches of diagonal line.
It is also desired to shorten treatment time to lower manufacturing cost and an improvement in throughput of a treatment apparatus and such an apparatus that can treat a large amount of substrates are being required.
In order to meet such requirements, sizes of plasma treatment apparatuses are being increased. For example, the size of the electrode in the apparatus as shown in FIG. 1 is increased. That is, in the case of the apparatus in FIG. 1, since the substrate is placed directly on the electrode, the simplest way for improving the treatment ability is to increase the area of the electrode. In such plane parallel plate type plasma treatment apparatus, however, the increase in the electrode size means an increase of the apparatus size, which is a problem for users who desire to treat a large substrate with a smaller apparatus taking a small floor space.
In addition to that, the increase of the electrode area hampers an uniformity of density of plasma that contacts to the surface of the substrate to be treated (discharge condition is ununiform), causing a problem that uniform plasma treatment cannot be implemented.
As a means for solving such problems, a method to dispose a plurality of plane parallel plate electrodes within a reaction chamber and to place a substrate to be treated on the electrode has been proposed, but it causes a problem of complicating the apparatus structure. Furthermore, even by this method, plasma discharge is still remained to be ununiform due to the increase of the area and uniform plasma treatment cannot be implemented.
As a means for solving such problems, an apparatus for carrying out plasma gaseous reactions has been devised (the apparatus is referred to as a positive column plasma treatment apparatus hereinafter). It has a structure in which plasma discharge is confined only in an area where substrates are placed within a reaction chamber, a frame called a container having a shape of surrounding the substrate from the four sides is disposed around the substrate to suppress the plasma from spreading to the inner wall of the reaction chamber, electrodes are separated from the substrate, parts of the frame not surrounded above and under the frame are covered by the electrodes and a space in which the substrates are placed is completely separated from the reaction chamber by the frame and the two electrodes to confine the plasma therein.
That is, the basic idea of this apparatus is to utilize a positive column area of a plasma glow discharge and to perform plasma treatment by disposing a plurality of substrates to be treated within the positive column area. A plasma CVD method utilizing this positive column is described in Japanese Patent Laid-open Nos. 59-52834 and 59-52835 which the applicant of the present invention applied.
FIG. 2 shows this method. As shown in the figure, a plurality of substrates 3 are placed between a pair of electrodes 7 and 1 by means of susceptors 10. The susceptors 10 are provided as integral parts of a container 13 and handling of the substrates is performed by inserting/taking the container to/out of a reaction chamber. To simplify its explanation, the figure only shows around a plasma discharge area. In this method, electrode shields 11 and 12 are provided around the electrodes to effectively utilize the positive column, and a reaction area is defined by the electrode shields and the peripheral wall of the container 13 to confine plasma not to discharge out of this area.
The apparatus has a structure in which the substrates are placed vertically and are surrounded by the frames, so that film may be formed on a plurality of substrates in the same time; this apparatus is an effective apparatus for solving the aforementioned problems.
However, a new problem arose also in such apparatus. In such method, in order to improve plasma treatment ability and to reduce occupied volume of the apparatus, the gaps between the substrates and the gaps between the substrate susceptors and the electrodes are narrowed down as much as possible. In the prior art method, however, the narrowed-down gaps unstabilized plasma discharge, causing the discharge to be extinguished or ununiformed and hampering uniform plasma treatment.
As well known methods for supplying discharge power to the electrodes in the positive column plasma treatment apparatus, there are following two methods as shown in FIG. 3. The method shown in FIG. 3a explains a double power source method for supplying two powers independently to each of a pair of electrodes 1 and 7 through the intermediary of matching module 9 and FIG. 3b explains a balun method for supplying one power to each of the pair of electrodes through the intermediary of the matching module 9 and balun 8.
In these power supplying methods, a blocking capacitor is inserted between the electrodes and the matching modules to apply self-bias on the substrate.
However, in the case of the positive column plasma treatment apparatus, the self-bias is not applied to the electrodes so much regardless of the existence of the blocking capacitor, even if either of the power supplying methods is adopted. Naturally no self-bias is applied when the blocking capacitor is removed.
At this time, plasma space potential is about Vpp/4 to Vpp/2 of a difference of two peak voltage values of applied power Vpp and when the container is assumed to be equal to earth potential, the container 13 receives sputtering of energy equivalent to the plasma space potential. It is the same when the substrate is conductive and is equal to the earth potential and in this case, coating films on the substrate 3 is susceptible to a large damage.
Even when the container 13 is kept in the earth potential, the surface of the substrate 3 is in a condition of being electrically insulated from the container 13 when the substrate 3 is electrically insulated from the container 13, so that the surface is turned into a condition of having a certain floating voltage. At this time, since a difference between the plasma space potential and the floating potential is smaller than that between the plasma space potential and the earth potential, the damage on the coating film is less as compare to the case when both container and substrate are kept in the earth potential.
Even in this case, however, plasma conditions change at the electrically insulated substrate and the container in the earth potential and at a boundary with the susceptors in particular because plasma near the substrates and that near the susceptors is influenced by the substrates and the susceptors, so that thickness and quality of a film differ near the edge of the substrate and at the middle part of the substrate; the uniformity of the thickness and quality of the film is impaired.
On the other hand, in the case of the positive column plasma treatment apparatus, since the container contacts partially with the reaction chamber, heat of the substrate easily escapes to the reaction chamber and heating and hot insulation cannot be easily implemented. Moreover, an impairment of uniformity of the temperature is a problem in heating a substrate in some container structures.
Furthermore, since the positive column plasma treatment apparatus can treat many substrates, a volume of plasma has to be increased and to this end, the gap between electrodes have to be widened, causing such problems that the size of the apparatus is increased and a large RF power is necessary.
Also in the case of the prior art positive column plasma treatment apparatus as shown in FIG. 2, when the substrate holding gaps and the gap between the electrodes and the susceptors are narrowed down, the susceptors and the surface of the substrates become paths for flowing current between the electrodes 7 and 1. This state is shown in FIG. 2 by a wave line 15. Due to that, even when plasma discharge has been observed in the beginning of the discharge and the plasma discharge is stopped in a state when there is no reflected power yet for inputted power, reflected power is not observed. Moreover, the plasma discharge stays only very near the upper and lower electrodes and current flows on the surface of the substrate susceptors during that, causing no plasma discharge near the substrates. Due to that, barely no inputted power is supplied to the gas, thereby hampering plasma treatment and causing an ununiform treatment.
In addition to that, in the case of the aforementioned apparatus, since there is a difference in degrees of plasma treatment on the substrate to be treated, normally a periphery of the substrate by 5 to 10% of length of the substrate size cannot be used. That is, in a case of plasma CVD apparatus for example, a coating film is formed on a substrate which is larger than what is originally needed and then its periphery is cut. Due to that, the area of the substrate has to be increased in performing plasma treatment, thereby increasing the apparatus size. The large substrate size causes an increase in material costs of the substrate and others.
Accordingly it is an object of the present invention to solve the aforementioned problems, i.e. to reduce the impairment on coating films or substrates, to improve the uniformity of thickness and quality of the coating films and to improve the substrate heating efficiency when the coating films are formed or plasma treatment is performed, by providing a plasma treatment apparatus having a novel structure that allows to treat a large substrate to be treated in a short treatment time and to perform uniform plasma treatment.